FIG. 1a schematically illustrates a conventional alternating current (AC)-direct current (DC) conversion system 100a employing an optocoupler 144 for voltage feedback. The system 100a includes a full bridge rectifier 108 receiving an AC input, and configured to output a pulsed DC signal that is operatively coupled to a primary side 116 of a transformer 112 (e.g., a flyback transfer). A secondary side 120 of the transformer 112 is operatively coupled to a diode 124, which outputs a DC voltage Vout.
The DC output Vout is controlled by controlling an operation of the transformer 112. The transformer 112 is controlled by controlling a switching of a MOSFET 138 that is operatively coupled between the primary side 116 of the transformer 112 and ground GND 1. For example, when the MOSFET 138 is turned on, energy is stored in the primary side coil of the transformer 112, whereas when the MOSFET 138 is turned off, the stored energy is transferred to the secondary side coil of the transformer 112, which is then rectified by the diode 124. The switching of the MOSFET 138 is controlled by a controller 130. The controller 130 includes a voltage feedback module 146 configured to receive a feedback of the output voltage Vout through a voltage sensor 140 and the optocoupler 144. The voltage feedback module 146, using the feedback received from the optocoupler 144, controls the switching of the MOSFET 138. During operation, the controller 130 regulates the output voltage Vout such that Vout tracks a reference voltage Vref.
The ground GND 1 is usually not a true earth ground (e.g., ground GND 1 may be a virtual ground, as GND 1 sees AC voltage during one half of each AC cycle), whereas a ground GND 2 (on the secondary side 120 of the transformer 112) may be a true earth ground. For this and other operational reasons (e.g., safety reasons), it may be desired to electrically isolate the primary side 116 and the secondary side 120. The optocoupler 144 couples the primary side 116 and the secondary side 120 using light (e.g., using light from a light emitting diode (LED) included in the optocoupler 144), but does not electrically connect the primary side 116 and the secondary side 120. Thus, the optocoupler 144 provides electrical isolation in the voltage feedback loop, i.e., provides electrical isolation between the controller 130 (coupled to the primary side 116) and the voltage Vout (in the secondary side 120 of the transformer 112).
FIG. 1b schematically illustrates a conventional voltage feedback system 100b that may be used in the AC-DC conversion system 100a of FIG. 1. More specifically, FIG. 1b illustrates the voltage sensor 140 and the optocoupler 144 of FIG. 1a in more detail. As shown in FIG. 1b, the voltage sensor 140 includes an operational amplifier 140a and a capacitor 140b forming a simple integrator circuit. The voltage sensor 140 outputs an error signal Verror that is a function of a difference between the output voltage Vout and the reference voltage Vref (i.e., Vout−Vref)). The optocoupler 144, which is operatively coupled to the voltage sensor 140, includes an LED 144a optically coupled to a photodetector 144b. The optocoupler 144 outputs a feedback signal that is proportional to the input to the optocoupler (Vout−Vref). Thus, the feedback signal is a function of (Vout−Vref), and the feedback signal is used by the voltage feedback module 146 to control the switching of the MOSFET 138.
FIG. 1c schematically illustrates a conventional AC-DC conversion system 100c employing a first optocoupler 144 for voltage feedback and a second optocoupler 154 for current feedback. Several components of the system 100c are similar to the corresponding components of system 100a, and are identified by the same identification labels in FIGS. 1a and 1c. In addition to the components identified in FIG. 1a, the system 100c of FIG. 1c also includes a current sensor 150 configured to measure a load current in the secondary side 120 of the transformer 112, and a second optocoupler 154 operatively coupled to the current sensor 150. The current sensor 150 and the optocoupler 154 are configured to provide feedback of the secondary side load current to a current feedback module 156 included in the controller 130. Similar to the optocoupler 144, the optocoupler 154 provides electric isolation between the primary and secondary sides of the transformer 112 in the current feedback loop.
In case a fault occurs in the secondary side 120 of the transformer 112 (e.g., short circuit or overloading on the secondary side 120), the secondary side load current usually increases (e.g., is several times the usual or normal secondary side load current). This increase in the load current is sensed by the current sensor 150, and transmitted to the current feedback module 156 in the controller 130 through the optocoupler 154. Upon detecting such a fault condition, the current feedback module 156 controls the MOSFET 138 to switch off the transformer 112, thereby preventing the transformer 112 from transferring energy from the primary side 116 to the secondary side 120 until the fault condition is cleared.
Thus, two different optocouplers (e.g., optocouplers 144 and 154) are used to feedback voltage and current sensing signals from the secondary side 120 to the primary side 116, and provide electrical isolation in the voltage and current feedback loop (i.e., provide electrical isolation between the primary side 116 and secondary side 120 of the transformer 112).